Al-Qaeda terrorists came within 45 days of attacking the New York subway system with a lethal gas similar to that used in Nazi death camps. They were stopped not by any intelligence breakthrough, but by an order from Osama bin Laden's deputy, Ayman Zawahiri. And the U.S. learned of the plot from a CIA mole inside al-Qaeda. These are some of the more startling revelations by Pulitzer Prize-winning author Ron Suskind, whose new book The One Percent Doctrine is excerpted in the forthcoming issue of TIME. It will appear on Time.com early Sunday morning.

U.S. intelligence got its first inkling of the plot from the contents of a laptop computer belonging to a Bahraini jihadist captured in Saudi Arabia early in 2003. It contained plans for a gas-dispersal system dubbed "the mubtakkar" (Arabic for inventive). Fearing that al-Qaeda's engineers had achieved the holy grail of terror R&D — a device to effectively distribute hydrogen-cyanide gas, which is deadly when inhaled — the CIA immediately set about building a prototype based on the captured design, which comprised two separate chambers for sodium cyanide and a stable source of hydrogen, such as hydrochloric acid. A seal between the two could be broken by a remote trigger, producing the gas for dispersal. The prototype confirmed their worst fears: "In the world of terrorist weaponry," writes Suskind, "this was the equivalent of splitting the atom. Obtain a few widely available chemicals, and you could construct it with a trip to Home Depot – and then kill everyone in the store."

The device was shown to President Bush and Vice President Cheney the following morning, prompting the President to order that alerts be sent through all levels of the U.S. government. Easily constructed and concealed, mass casualties were inevitable if it could be triggered in any enclosed public space.

Having discovered the device, exposing the plot in which it might be used became a matter of extreme urgency. Although the Saudis were cooperating more than ever before in efforts to track down al-Qaeda operatives in the kingdom, the interrogations of suspects connected with the Bahraini on whose computer the Mubtakkar was discovered were going nowhere. The U.S. would have to look elsewhere.

Conventional wisdom has long held that the U.S. has no human intelligence assets inside al Qaeda. "That is not true," writes Suskind. Over the previous six months, U.S. agents had been receiving accurate tips from a man the writer identifies simply as "Ali," a management-level al-Qaeda operative who believed his leaders had erred in attacking the U.S. directly. "The group was now dispersed," writes Suskind. "A few of its leaders and many foot soldiers were captured or dead. As with any organization, time passed and second-guessing began."

And when asked about the Mubtakkar and the names of the men arrested in Saudi Arabia, Ali was aware of the plot. He identified the key man as Bin Laden's top operative on the Arabian Peninsula, Yusuf al Ayeri, a.k.a. "Swift Sword," who had been released days earlier by Saudi authorities, unaware that al-Ayeri was bin Laden's point man in the kingdom.

Ali revealed that Ayeri had visited Ayman Zawahiri in January 2003, to inform him of a plot to attack the New York City subway system using cyanide gas. Several mubtakkars were to be placed in subway cars and other strategic locations. This was not simply a proposal; the plot was well under way. In fact, zero-hour was only 45 days away. But then, for reasons still debated by U.S. intelligence officials, Zawahiri called off the attack. "Ali did not know the precise explanation why. He just knew that Zawahiri had called them off."

The news left administration officials gathered in the White House with more questions than answers. Why was Ali cooperating? Why had Zawahiri called off the strike? Were the operatives planning to carry out the attack still in New York? "The CIA analysts attempted answers. Many of the questions were simply unanswerable."

My preliminary thoughts are these: whatever you happen to feel about George Bush, one thing is clear: When it comes to defending the homeland against al Qaeda, he has not hesitated to act in the decisive way he and his advisors see fit. To that end, he has proven himself unafraid to use substantive military force, and largely immune to the opinions of both the western media and international elites. Whether or not this factored into al Qaeda’s thinking is dubious. But I have long believed that one of the reasons we haven’t seen the kind of attacks here that we see in, say, Israel, is that the US, should it ever decide to go on the full offensive, cannot be restrained—particularly if public opinion shifts toward a desire to see the enemy eradicated, even if doing so requires a shift in the collective moral calculus of the nation.